What causes the crescent sign of the femoral head on radiography of the hip?
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Answer:
The crescent sign is a thin subarticular lucency running parallel to and just beneath the articular cortex of the femoral head. It is a subchondral fracture through weakened necrotic trabecular bone at the junction with viable bone, and its presence establishes that the head has structurally failed even though the spherical contour may still be intact. It is best seen on the frog-leg lateral projection, because the fracture lies in the anterosuperior weight-bearing segment and is obscured by overlapping cortex on the frontal view; a normal anteroposterior radiograph therefore does not exclude it. In the ARCO classification the sign defines stage 3, subdivided by whether the head has begun to flatten, and it is the point at which joint-preserving options such as core decompression become unreliable and collapse becomes the expected course. The most important differential is subchondral insufficiency fracture in an older or osteoporotic patient, in which MRI shows a low signal line that is irregular, convex toward the articular surface and accompanied by extensive marrow oedema, in contrast to the smooth serpiginous double line sign that demarcates the necrotic segment in osteonecrosis. Rapidly destructive coxarthrosis and septic arthritis are excluded by joint space loss and clinical features. CT resolves an equivocal fracture line and MRI defines the size of the necrotic segment, which is the strongest predictor of collapse.
Why is it called so?
The subchondral fracture line follows the curve of the articular surface, so the trapped gas and lucent cleft form a crescent under the cortex.
Pathophysiology
After interruption of the femoral head blood supply, marrow and osteocytes die but the trabecular scaffold initially remains. Creeping substitution at the reactive interface resorbs necrotic trabeculae before new bone is fully deposited, so a mechanically weak zone develops immediately beneath the subchondral plate. Repetitive weight-bearing shear across this zone produces a fatigue fracture separating the necrotic segment from the underlying living bone. Gas released into the fracture cleft and the fracture itself account for the lucency. Because the overlying cartilage retains its nutrition from synovial fluid, it stays intact until the supporting bone collapses beneath it.
Alternative names: Subchondral crescent; rim sign of the femoral head
Other associated named signs: Double line sign on T2-weighted MRI, the bite sign of segmental collapse, and the half-moon sign
References
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